Tire Size Box

195/40R18 vs 195/45R18

195/45R18 compared with 195/40R18

+3.2% diameter, 0.0" width

195/45R18 is 24.91" tall vs 24.14". More than the 3% rule of thumb, so check carefully before switching.

24.1"18" rim3.1"7.7" wide
195/40R18 (solid) compared with 195/45R18 (dashed outline), drawn to scale

With 195/45R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

195/45R18 is +0.77 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

195/40R18195/45R18Difference%
Overall diameter24.14"24.91"+0.77"+3.2%
Section width7.68"7.68"0.00"0.0%
Sidewall height3.07"3.45"+0.38"+12.5%
Circumference75.84"78.26"+2.41"+3.2%
Revolutions per mile835.4809.7−25.7−3.1%

Speedometer difference

If your vehicle came with 195/40R18 and you fit 195/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 195/40R18 or 195/45R18?
195/45R18 is taller by 0.77 inches (24.91" vs 24.14"). 195/40R18 is the same width.
Can I replace 195/40R18 with 195/45R18?
It is not recommended without checking carefully. The diameter difference is +3.2%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 195/45R18 change my speedometer?
When your speedometer reads 60 mph with 195/45R18 fitted (calibrated for 195/40R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 195/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.38 inches of ground clearance at the axle.

Other comparisons